Field of the Invention
The present invention relates to a display device and a method for manufacturing the same, and more particularly, to a shift register of a display device, which uses an oxide transistor.
Discussion of the Related Art
Flat display devices include a liquid crystal display (LCD) device using liquid crystal, an organic light emitting diode (OLED) display device using an organic light emitting diode, an electrophoretic display (EPD) device using electrophoretic particles, and so on.
A flat display device includes a display panel for displaying an image through a pixel array including pixels, each of which is independently driven by a thin film transistor (TFT), a panel driver for driving the display panel, and a timing controller for controlling the panel driver. The panel driver includes a gate driver for driving gate lines of the display panel and a data driver for driving data lines of the display panel.
The gate driver includes a shift register which outputs scan pulses for individually driving the gate lines of the display panel. The shift register typically includes a plurality of stages dependently connected to one another. Each stage includes a plurality of TFTs. The output of each stage is supplied as the scan pulse to each gate line while being supplied as a carry signal for controlling another stage. Recently, the gate driver is integrally formed when the TFT array of the pixel array is formed by mainly using a gate in-panel (GIP) technique.
An oxide semiconductor TFT (hereinafter, referred to as an oxide TFT) is recently spotlighted as a TFT included in a display panel since the oxide TFT has higher mobility than that of an amorphous silicon TFT, and can be manufactured at a lower temperature than a poly-silicon TFT. Thus, such a display panel including an oxide semiconductor TFT is suitable for large-scale applications. However, the oxide TFT is sensitive to light and, as such, device characteristics may change when being exposed to light.
FIG. 1 is a voltage-current graph illustrating variable characteristics of a threshold voltage in accordance with time of light irradiation of a general N-type oxide TFT.
Referring to FIG. 1, as an active layer irradiated with light, an oxide active layer deteriorates and, as such, the threshold voltage Vth of the N-type oxide TFT shifts to a negative value. As light irradiation continues, the oxide active layer further deteriorates and, the threshold voltage Vth further shifts in the negative direction, as shown in FIG. 1.
In detail, although an off-voltage is applied as a gate voltage of the N-type oxide TFT mainly used as a shift register, the gate voltage is not lower than a low voltage applied to a source electrode. When an off-voltage is applied to the gate of the oxide TFT, the oxide TFT should be logically turned off. However, as the threshold voltage of the N-type oxide TFT shifts in the negative direction due to light deterioration, a leakage current, which flows because a voltage Vgs between the gate and source electrodes is greater than 0V (Vgs>0V), increases, and, as such, the shift register may not output a normal waveform.
For example, when the threshold voltage shifts in the negative direction due to light deterioration of the N-type oxide TFT, a leakage current of a node control part for controlling a pull-up TFT is generated and, as such, the voltage of a control node is distorted. Accordingly, output faults, such as a distorted waveform of the scan pulse output by the pull-up TFT or an uneven output of the scan pulse, may be generated.